This project is designed to determine the relationship betwen DNA repair and mutagenesis in Drosophila melanogaster. Three approaches are being taken: (1) A mutant which increases the mutation frequency (a mutator) has been identified, mapped, and characterized. This mutator blocks repair of chromosome breaks specifically in oocytes, thereby allowing a previously undescribed repair process to be observed. In this process broken chromosomes are "healed", allowing the recovery of terminal deletions. (2) The interaction of DNA repair-defective mutants and transposable elements has been observed in double mutant combinations. None of the repair-defective mutants examined to date influence the rates of transposon-induced mutation or recombination, although mutants at the mei-41 locus prevent the transmission of transposon-bearing chromosomes. (3) Aneuploidy is being examined as a genetic endpoint. Chemicals that induce aneuploidy are being identified as probes to investigate mitosis and meiosis.